Many industries have a commercial need to monitor the concentration of particular constituents in a fluid. In the health care field, individuals with diabetes, for example, have a need to monitor a particular constituent within their bodily fluids. A number of systems are available that allow people to test a body fluid, such as, blood, urine, or saliva, to conveniently monitor the level of a particular fluid constituent, such as, for example, cholesterol, proteins, and glucose. Individuals with diabetes, a pancreatic disorder characterized by insufficient insulin production, prevents the proper digestion of glucose, have a need to carefully monitor their blood glucose levels on a daily basis. A number of systems that allow people to conveniently monitor their blood glucose levels are available. Such systems typically include a test strip where the user applies a blood sample and a meter that “reads” the test strip to determine the glucose level in the blood sample.
Among the various technologies available for measuring blood glucose levels, electrochemical technologies are particularly desirable because only a very small blood sample may be needed to perform the measurement. In amperometric electrochemical-based systems, the test strip typically includes a sample chamber that contains reagents, such as glucose oxidase and a mediator, and electrodes. When the user applies a blood sample to the sample chamber, the reagents react with the glucose, and the meter applies a voltage to the electrodes to cause a redox reaction. The meter measures the resulting current and calculates the glucose level based on the current. Other systems based on coulometry or voltametry are also known.
Because the test strip includes a biological reagent, every strip manufactured is not reproducible with the exact same sensitivity. Therefore, test strips are manufactured in distinct lots and data particular to that lot is often used as a signal by the meter's microprocessor to assist in accurately performing the meter calculation. The data is used to help accurately correlate the measured current with the actual glucose concentration. For example, the data could represent a numeric code that “signals” the meter's microprocessor to access and utilize a specific set of stored calibration values from an on-board memory device during calculation.
In past systems, the code particular to a specific lot of strips has been input into the meter manually by the user, or connected through some type of memory device (such as a ROM chip) packaged along with test strips from a single manufacturing lot. This step of manual input, or connection by the user, adds to the risk of improperly inputting the wrong code data. Such errors can lead to inaccurate measurements and an improper recording of the patient's history. Past systems have also included bar-code readable information incorporated onto individual strips. Individually imprinting a particular bar-code on each strip adds significant manufacturing costs to the strip production and requires the additional expense of a bar-code reader incorporated within the meter in order to obtain the information.
It should be emphasized that accurate measurements of concentration levels in a body fluid, such as blood, may be critical to the long-term health of many users. As a result, there is a need for a high level of reliability in the meters and test strips used to measure concentration levels in fluids. Thus, it is desirable to have a cost effective auto-calibration system for diagnostic test strips that more reliably and more accurately provides a signaling code for individual test strips.
Embedding strip lot calibration information onto individual test strips which is readable by the instrument (meter), eliminates the need for the user to match the meter's lot calibration to the vial of strips. No longer needing to rely on the user to properly calibrate the meter's lot code removes the possibility of user error for this critical step.
Although user technique error is eliminated from automatically calibrated systems, the system is still subject to potential instrument read errors due to normal variations in production of strips and instruments. These systems are susceptible to erroneously read calibration codes whether the lot code is embedded electrically, mechanically, optically or otherwise. Abating or at least reducing the chance for a instrument read error will greatly enhance the reliability of the system.
Complete elimination of read error is possible, but is limited by the number of useful data bits that can be encoded on the small strip structure. Minimizing read errors is possible without sacrificing the number of available data bits for auto-calibration, but requires proper mathematical arrangement of and numbering of lot codes. A combination of the techniques taught in the invention described below can be employed to vary the read error rejection up to 100%, depending on a predetermined acceptable level of error rejection.
On-strip coding is a relatively new concept in glucose testing, which has the potential to greatly improve the accuracy of glucose readings. Systems that do not protect against test strip coding errors, either through detecting and rejecting strips which contain errors, or correcting read errors, will be subject to suboptimal performance and may produce inaccurate glucose readings.